The complement system forms the central core of innate immunity but also mediates a variety of inflammatory diseases. Recent studies with human subjects and animal models demonstrated that complement component C3a plays a novel and important role in the pathogenesis of asthma. The cellular and molecular mechanism by which C3a modulates asthma, however, remains unknown. Our preliminary studies demonstrated that G protein coupled receptor (GPCR) for C3a (C3aR) are expressed in human mast cells and that C3a causes rapid mast cell degranulation/leukotriene C4 generation and delayed NF-?B activation/chemokine generation;pathways that coordinately regulate airway hyperresponsiveness (AHR) and lung inflammation, two major features of asthma. Our preliminary studies also suggest new concepts in the regulation of C3aR function in mast cells, which may provide novel targets for differential regulation of AHR and lung inflammation. Receptor phosphorylation by G protein coupled receptor kinases (GRKs) and the subsequent recruitment of [unreadable]-arrestin provides an important mechanism for desensitization of GPCRs. We made the novel observation that C3aR phosphorylation, which inhibits mast cell degranulation, provides a stimulatory signal for chemokine gene expression. Based on these findings, we hypothesize that C3aR phosphorylation by distinct GRKs differentially regulates AHR and lung inflammation by inhibiting mast cell degranulation but stimulating chemokine gene expression. Three specific aims are proposed to test this hypothesis. In aim #1, we will test the hypothesis that phosphorylation of C3aR at distinct site by different GRKs inhibits mast cell degranulation but promotes NF-?B activation/chemokine generation. In aim #2, we will test the hypothesis that phosphorylated C3aR associates with both 2-arrestin and PSD-95/Dlg/Zo1 (PDZ) domain containing proteins to inhibit and stimulate NF-?B activity, respectively. In aim #3, we will modulate allergen-induced AHR and lung inflammation in vivo by targeting C3aR phosphorylation in mast cells. Collectively, these studies will generate significant new information on how C3aR signaling modulates asthma and may offer novel therapeutic approaches for the treatment of asthma and airway inflammation. Narrative: Asthma is a complex airway inflammatory disease characterized by bronchoconstriction, airway hyperresponsiveness (AHR) and inflammation. Approximately 17 million Americans are estimated to have asthma, one third of them children. In recent years, asthma prevalence and severity have been increasing dramatically world-wide. Mast cells present in the lung release mediators that cause the symptoms of asthma. This proposal is based on the identification of a new molecule that regulates mast cell function in asthma. We believe that proposed studies will generate significant new information on the regulation of mast cells and may offer novel therapeutic approaches for the treatment of asthma.